Etherianus, Hugh and Leo, brothers, Tuscans by birth, employed at the court of Constantinople under the Emperor Manuel I (Comnenus, 1143-1180). Their name is spelled in various ways: Aetherianus, Heterianus, Eretrianus, etc. Leo is of little importance. We know from his brother (Adv. Graec., I, 20) that he was "occupied in translating the imperial letters", evidently an interpreter for Latin correspondence. Hugh, who does not seem to have held any official post at court, but was a very learned theologian, had many opportunities of discussing the questions at issue between the Orthodox and Catholics (so he tells us: Adv. Graec., Praef. I., Migne, P.L., CCII, 165). As a result of these disputes he wrote a work in three books: "De haeresibus quas Graeci in Latinos devolvunt, sive quod Spiritus Sanctus ex utroque Patre et Filio procedit" (P.L., CCII, generally quoted as "Adv. Graecos"). This work, the first exhaustive and scientific defense of the Filioque, was composed in both languages, Latin and Greek. The author sent copies to the Orthodox Patriarch of Antioch, Aimerikos, and to Pope Alexander III (1159-1181), whose letter of acknowledgment is still extant (Ep. xlix, Baronius, an. 1177, n. 37, 38). Hugh Etherianus by this treatise obtains a very important place among Catholic controversialists against the Eastern Church. It appears that the emperor, who was well disposed towards Latins, had suggested that he should write it, having asked him whether they have "any authorities of saints who say that the Holy Ghost proceeds from the Son" (ib., Praef. I, CCII, col. 165). Hugh had used his knowledge of Greek and his opportunities of studying their Fathers so well that he was able to produce texts from nearly all the recognized authorities on both sides. He quotes especially Sts. Athanasius, Cyril of Alexandria, Basil, Gregory Nazianzen, Chrysostom, John Damascene, etc. From the Latins he produced witnesses from Sts. Augustine, Jerome, Gregory I, Ambrose, Hilary. He was also well acquainted with the writings of his adversaries and quotes Photius, Nicetas of Thessalonica, Theophylactus of Achrida, etc. The Latin version is very corrupt and untrustworthy. There are also some incorrect expressions noted by the later editors, such as that God the Father is the cause of the Son (this is a concession to the Greeks that was, however, tolerated by the Council of Florence; Denzinger, Enchiridion, n. 586). Nevertheless, since it was written this work has been the foundation of nearly all Latin controversy with the Greeks. St. Thomas Aquinas used it for his "Opusc. I, contra errores Graecorum" and Cardinal Bessarion refers to it with great praise (Ep. ad Alex., P.L., CLXI, 328). Hugh Etherianus also wrote a treatise "De regressu animarum ab inferis", in answer to a petition of the clergy of Pisa, and (probably) a short work "De Graecorum malis consuetudinibus". A "Liber de immortali Deo", written by him, is lost.